The present invention generally relates to electro-chemical machining (ECM). More particularly, this invention relates to a method of performing ECM in-situ of a portion of a dovetail slot, for example, edge regions of slots within turbine wheels employed in turbomachines, including but not limited to gas turbines used in power generation.
In the hostile operating environments of gas turbine engines, the structural integrity of turbine rotor wheels, buckets, and other components within their turbine sections is of great importance in view of the high mechanical stresses that the components must be able to continuously withstand at high temperatures. For example, the regions of a turbine wheel forming slots into which the buckets are secured, typically in the form of what are known as dovetail slots, are known to eventually form cracks over time, necessitating monitoring of the wheel in these regions. In some wheel configurations, non-limiting examples of which include the stage 1, 2, and 3 wheels of the General Electric 9FB gas turbine, cooling of the buckets and wheel perimeter is assisted by the presence of a cooling slot located near the perimeter of the wheel and into which the dovetail slots extend. Over extended periods of time under the severe operating conditions of a wheel, cracks may form at common edges formed where the dovetail slots and cooling slot intersect. Configuring of the cooling slot geometry to reduce the likelihood of such cracks is desirable in order to improve expected life of a turbine wheel.
While a turbine rotor can be completely disassembled to gain access to its individual wheels, ECM techniques that can be performed with limited disassembly are preferred to minimize downtime, such as to fit within outage schedules of a gas turbine employed in the power generating industry. However, access to the cooling slot is very limited, and any ECM technique must address the difficulty of bringing the tool into stable proximity to the edges being rounded.
Currently, cooling slots of gas turbine engines are generally rounded by mechanical grinding followed by a finishing process, such as BPP (blend, polish, peen). These methods involve using a bit to remove material at the edge of the cooling slot and then blending and/or polishing the edges to obtain the desired radius of the intersection edges. However, a desired radius is often difficult to achieve if the grinding was performed by mechanical means. Furthermore, BPP methods may fail to remove all of the cracks in the cooling slots.
Therefore, it would be desirable if a method existed by which sharp edges prone to cracks on a turbine wheel, particularly edge regions of slots within the wheel, could be rounded to a desired radius with minimal polishing and/or blending using ECM. It would also be desirable if such a process were able to be performed without necessitating complete disassembly of a turbine rotor to gain access to its individual wheels.